1. Field of the Invention
The present invention relates to a new directionally-linked surface useful for the alignment of liquid crystals. Methods of preparing this new surface are also described, as is a preferred method wherein a polymerizable self-assembled and high resolution-patternable sorbed layer is polymerized by polarized radiation.
2. Discussion of the Prior Art
Flat panel liquid crystal (LC) display devices typically require substrates which provide for the uniaxial orientation of liquid crystalline molecules. Currently, such alignment is achieved by initially spin coating a polymer (polyamide, polyimide, etc.) on a substrate followed by mechanical rubbing of the polymer surface with cotton, rabbit fur, etc. This technique has inherent problems, however, in that it is difficult to obtain a predictably uniform alignment over large areas. In addition, rubbing creates both charges and dust. Charges lead to the failure of LC devices due to, e.g., shorting of conducting surfaces, etc., and dust can provide defect sites. There is, therefore, a current industrial need for a surface that promotes uniaxial liquid crystal alignment (i.e., both planar alignment (an alignment orientation where the long axis or average director! of the liquid crystal is not perpendicular to the plane of the substrate surface), and homeotropic alignment, where liquid crystal molecules are perpendicular to the substrate surface) without the need to mechanically rub the surface of a substrate, and for a simple technique for producing such a surface. Colorless substrates that align liquid crystalline molecules are also desirable for high contrast applications, etc.
Current approaches for addressing the problems encountered with rubbed surfaces include the coating of Langmuir-Blodgett (LB) layers on substrates, and the polymerization or irradiation of substrates that have been coated with polymers with polarized light.
In the LB film approach, major difficulties are encountered which have not yet been overcome: (i) it is very difficult to scale-up an LB process for manufacturing purposes, and (ii) a useful aligning layer is fabricated only via the layer-by-layer deposition of monolayers onto a substrate at the air-water interface of an LB trough. Since the monolayer present at the air-water interface in an LB trough is not in its thermodynamically stable state, the aligning layer obtained does not have long term mechanical and thermal stability.
LB layers also generally contain inhomogeneities or domains of defects within the plane of the film. This makes it difficult to obtain the uniaxial alignment of a liquid crystal compound over a large area. Moreover, LB films are known to have a considerable degeneracy with regard to the tilt of the LB-forming molecules within the plane of the film. As a consequence, it is not possible to obtain specific pre-tilt directions at the substrate surface.
Hercules (U.S. Pat. No. 5,032,009; U.S. Pat. No. 4,974,941; U.S. Pat. No. 5,073,294; Nature, Vol 351, 49 May 1991; Liquid Crystals, Vol. 12, No. 5,869, 1992; Newsletter of the Int. Liq. Crystl. Soc., ("Liquid Crystals Today"), Vol. 4, No. 2 1994, all incorporated herein by reference) has reported the alignment of liquid crystals optionally containing dyes with polarized light and the preparation of a surface made of an isomerizable dye which is first dispersed in a polymer and subsequently irradiated with polarized light. Neither the liquid crystal itself, dye, nor host polymer is covalently bound to the substrate, and the aligning surface is unstable: heat and/or subsequent irradiation changes or erases any initially obtained orientation aligning effect.
Schadt et al (Jpn. J. Appl. Phys., Vol. 31, Pt 1, No. 7, p. 2155 (1992); EP 525,477; EP 525,473 and EP 525,478, all incorporated herein by reference) has also reported aligning surfaces prepared by the irradiation of polymers having, attached to the polymer backbone, pendant polymerizable groups, using polarized light. These surfaces require pre-polymerization, however, and, like those described above, are not covalently bound to a substrate surface. Moreover, when polymer layers like those of Schadt and Gibbons are made thin to lower the driving voltage, pinholes arise which cause defects and shorts.
Finally, Ichimura (Abstract from the Tawiguchi Conference, (Japan), 1994, incorporated herein by reference) has used polarized light to orient polymers bearing side chain azobenzenes. The polymers were applied to substrates using an LB technique and a spin-coating technique, and showed alignment of liquid crystals in contact therewith. However, these films, like those of Schadt and Gibbons, are not bound to the substrate surface, and they suffer from the general drawbacks discussed above for such films as well as those discussed regarding LB films.
Thus, there remains a need for a tough new, non-rubbed, non-LB based alignment surface which can be easily produced and used in liquid crystal devices which require alignment layers. The present invention provides such a surface.